1. Field of the Invention
The present invention relates to downhole anchor devices for use in eliminating the effects of reactive torque in directional drilling procedures.
2. Description of the Prior Art
Directional drilling is widely utilized today for the efficient exploitation of oil and gas fields, but requires great accuracy in drill orientation as to both azimuth and inclination or drift angle.
Straight sections of the bore are drilled with a simple rotary drill, and required alterations in the direction of the bore are achieved using a downhole mud motor drill of either the positive displacement or turbine type which is offset at a small angle relative to the drill string axis by means of a bent sub. The bent sub causes the drill to follow a path which curves away from the straight section of the bore above it, and steering is accomplished by rotation of the drill pipe at the surface to the desired azimuthal orientation of the bent section. The amount of deflection or inclination from the straight section of bore above is determined by the length of the curved path that is followed by the drill at the desired azimuth.
Once the downhole mud motor drill introduces a deviation from the vertical section of bore, however, reaction torque from the torque exerted on the drill bit by the mud motor is transmitted to the drill string and begins to force the drill bit away from its desired azimuthal orientation. So far, response to this difficulty has developed several survey instruments that are lowered on wirelines down the drill string to monitor changes in azimuth and inclination of the drill bit, relaying such information to a surface readout to enable correction at the surface. These devices are extremely expensive (currently $20,000 to $50,000 per job inside the U.S.A. and $50,000 to $100,000 per job elsewhere) and incur further costs due to the necessity of tripping them in and out of the drill string in order to add sections of drill pipe to the top of the drill string. The wireline companies make charges per foot of bore depth for each round trip of the wireline plus hourly rates for the truck, operator and a helper. Tripping of the wireline into and back out of the drill string at a depth, for example, of 5,000 feet sets drilling time back approximately 21/2 to 3 hours. Considering daily charges made by the companies providing the survey and readout equipment as well as the daily rate of the directional drilling company, the problem of reactive torque and consequent azimuthal drift of the drill bit rapidly becomes an insupportable financial burden.
The only device of which applicant is aware that is currently employed for the reduction of azimuthal drift is the kelly, which tool loses most of its efficacy by being located on the surface. The kelly attempts to eliminate such drift by gripping the top of the drill string against rotation. This allows reactive torque to twist or wind up the entire length of the drill string below the surface, which can extend for thousands of feet. In addition, the kelly restricts the depth which may be drilled in one operation. Most drilling operations have the capacity of drilling a 90-foot stand of drill pipe at one time. The kelly, however, requires that singles, or 30-foot sections of drill pipe, be separately added and drilled in order to regrip the drill string adequately. Since the kelly has little effect in reducing azimuthal drift, it does nothing to reduce the number of wireline trips required by the survey instrumentation or the cost thereby incurred.
Applicant is not aware of any prior art device or method that will adequately absorb reactive torque near its source downhole. In particular, applicant is not aware of any prior art device or method that will completely absorb reactive torque near its source downhole and positively prevent rotational travel or twisting of the drill string and hence positively prevent azimuthal drift in directional drilling, while at the same time allowing free, unrestrained axial movement of the drill string.
Applicant is aware of a number of prior art patents which disclose well drilling apparatus in which rollable wheels engage the wall of a well and are oriented so as to travel axially along the well as drilling occurs. These include U.S. Pat. Nos. to Karns 879,822, Wittich 969,233, Maher et al. 1,072,964, Blackwell 1,350,059, Mann 2,796,234, Brown 3,249,162, and Werner 3,729,057, and also French Pat. No. 2,275,633 to Bonet-Thirion. Most of these wheels are employed simply for alignment or stabilizing purposes. Only the Maher et al. and Brown disclosures show an intention that the wheels prevent rotational movement of the drilling tool in the well, but the tools of both Maher et al. and Brown are suspended in the well, on a cable in Maher et al. and on a tube rolled up on a surface drum in Brown. Neither of the suspended Maher et al. and Brown tools could possibly be used for directional drilling because directional drilling requires use of a conventional drill string in order to establish the desired azimuthal direction of the bent sub near the bottom of the string used to establish and achieve the desired azimuthal direction of drilling.
Although Maher et al. and Brown disclose the purpose of axially rolling wheels to be the prevention of rotation of the drilling tool, applicant has found by means of extensive testing that it is impossible to prevent rotational travel and deflection of portions of a drill string near its downhole end with rollable anchor devices such as wheels, even though the wheels are numerous and have specially designed serrated gripping peripheries specifically intended to prevent rotational travel of the drill string. Even a small amount of rotational travel and deflection of a directional drilling string near its downhole end will cause a major amount of horizontal offset of the directional bore as drilling proceeds further and further from the initial, usually vertical shaft. This is intolerable in directional drilling procedures. Applicant believes the reason gripping wheels are unable to prevent rotational travel and deflection as drilling proceeds is the moving or dynamic nature of the contact between the wheels and the well bore. With continuous downward movement of the drill string, the wheels are continuously moving from point to point along the wall of the well and are never at rest or staticaly located, and therefore are never able to obtain a fixed lock against the well wall. This is particularly the case where the wheels are engaged against a well casing, in which case the contact of the wheels against the casing is essentially a point or transverse line contact. This is an insufficient purchase for preventing rotational travel and deflection under the high reaction torque produced during directional drilling with a downhole hydraulic fluid or mud motor.
Applicant is aware of three prior art patents which disclose attempts to prevent rotation of a drilling tool under the influence of reaction torque by a static type of device. These are U.S. Pat. Nos. 1,112,946 to Turnbull, Taylor 1,870,697, and French Pat. No. 1,271,163 to Allard. In Turnbull, four solenoid armatures are projected magnetically outwardly into engagement with a metal casing wall, and the concept was that these would allow axial movement while resisting rotational movement. However, these solenoid armatures are just as slideable rotationally on the wall of the casing as they are axially, so that rotational travel would be inevitable under the influence of reaction torque. Taylor discloses a pair of axially serrated grippers pivotal outwardly into gripping engagement with the well bore under the influence of hydraulic pressure when a weighted upper part of the tool is released downwardly relative to the lower part of the tool in which the gripping members are mounted, which actuates a hydraulic cylinder to provide the necessary hydraulic pressure. The problem with this device is that the grippers are dovetailed to the tool for sliding movement, and this only allows a relatively short increment of travel of the drilling tool until drilling must be stopped, the grippers released by pulling upwardly on the weighted part of the tool, allowing the gripping members to slide downwardly in the dovetail tracks to a new position at which the tool can then be reactivated. Such repeated disengagement after short increments of drilling travel would cause azimuthal orientation to be lost in directional drilling. Additionally, the tools of both Turnbull and Taylor are cable suspended and hence could not possibly be used in directional drilling.
The underreamer apparatus in French Pat. No. 1,271,163 to Allard has the same deficiencies as the Turnbull device. Allard discloses pads hydraulically compressed against the wall of a well casing, with the apparatus like Turnbull being suspended on a cable and hence unusable in directional drilling. The gripping pads appear to be intended to secure the Allard tool against both axial and rotational movement, but if they were axially slideable in operation, they would necessarily also be rotationally slideable as for the apparatus disclosed in Turnbull.